Convoluted boots are used to seal lubricant within the open ends of constant velocity joints and to protect such joints from environmental conditions such as dirt and water. Typically, a convoluted boot, at its large opening, is connected by means of a first collar to the outer part of a constant velocity joint and, at the small opening at the other end, it is secured by means of a second collar on a driveshaft. Annular folds between the collars of the boot make it possible to follow every change in the angle of the driveshaft relative to the outer part of the constant velocity joint. The convoluted boot prevents the lubricant from escaping from the constant velocity joint and dirt from entering the joint.
Convoluted boots are presently produced from thermoplastic elastomers, and preferably elastomers based on polyester. At high articulation angles, the folds of the boot rub against one another which, when the joint rotates, can lead to the development of a great deal of noise and also to mechanical damage to the boot, due to the abrasion of the polymer material. Particularly, when the outside of the boot folds are wet, such boots commonly generate a particularly unpleasant type of noise in the form of a squeaking sound.
To prevent or reduce the development of noise and abrasion, several proposals have been made. For instance, it has been attempted to provide the outer surface of the boot with a special coating consisting of waxes and/or paraffinic oils and/or ester oils, thus generating a lubricating and noise-reducing effect when the convoluted boot is articulated and when, as a result thereof, there occurs a close contact between the folds. However, it has been found that this type of coating is not sufficient for providing long-term protection. Especially under wet and very dusty conditions, the coating is used up relatively quickly. In addition, there is the disadvantageous need for highly technological methods required for treating the outer surface of the boot.
Convoluted boots are produced by processing a granular, thermoplastic elastomer material. It has been proposed to work amide waxes and/or paraffinic oils into the boot wall via said granulate. In this regard, use is made of the insignificant ability of materials based on thermoplastic, elastomer polyester to receive this type of component, which leads to the components being separated on the boot surface where they form a coating similar to that described in the previous paragraph. This means that in this case, too, adequate, permanent adherence of the lubricating components is not ensured.
There exists an additional disadvantage in that the admixtures are also separated towards the inner boot surface, which, in the region of the clamping-in zones of the collar, especially at low temperatures, leads to friction locking problems between the boot, outer joint part, driveshaft and connecting elements.
Furthermore, there exists the risk of damaging emissions as a result of vaporisation and thermal decomposition of the added components in the course of the processing of the granulate. In particular, such undesirable emissions occur in cases when phosphoric acid esters are worked into the granulate.
Furthermore, it was proposed to work carbonic acid ester into a polyester material. However, it has been found that with this kind of additive, the polyester material only begins to swell, but it is not possible to achieve tribological conditions at the outer boot surface.
DE 100 36 447 A1 describes a flexible artificial resin sleeve and a method of producing same. The artificial resin sleeve which, in particular, is used in constant velocity joints at the driveshafts of motor vehicles, substantially consists of a thermoplastic polyester elastomer resin. To prevent the above-described development of noise, it was proposed to add mineral oil or vegetable oil to the thermoplastic elastomer resin to prevent the development of noise at the start of operation and at a later stage, even under conditions where the joint with the sleeve is rotated at large angles. It was also the object of the above publication that the noise-reducing effect of the sleeve described therein should continue over a long period of time. However, it has been found that the addition of large amounts of mineral oils or vegetable oils led to a decrease in the friction coefficient in the friction locking connections, which is due to the fact that a lubricating film is formed not only in the regions of the folds, but also in the region of the connections with the driveshaft and the outer joint part of the constant velocity joint.